1. Field of Use
This invention relates generally to control systems for multi-speed shiftable power transmissions such as are used in off-highway trucks. In particular, it relates to an electronic logic controller for such systems which receives relevant signal information and provides control signals to effect automatic shifting, provided predetermined operating conditions are met.
2. Description of the Prior Art
U.S. Pat. Nos. 3,805,640 and 4,015,488, owned by the assignee of the present application, each disclose a control system for a multi-speed shiftable power transmission wherein a plurality of clutches in the transmission are hydraulically operated to effect shifting. Each clutch is operated by a separate individually actuatable solenoid valve, and these solenoid valves are actuated in proper sequence to effect shifting by an electronic controller which receives signal information from a speed range selector switch and other appropriate sources. These prior art systems are complex and costly because each requires a plurality of clutch solenoid valves, redundant hydraulic circuitry therefor, and redundant electrical circuitry in the electronic controller in order to effect proper sequential operation of the clutch solenoid valves. Furthermore, since each shift from one speed range to another requires sequential operation of several solenoid valves, the time to effect each shift is unduly prolonged.
U.S. patent application Ser. No. 73,508 filed Sept. 7, 1979 by Behlke, Black and Steinhagan, and owned by the assignee of the present application, discloses an improved multi-speed shiftable power transmission wherein the several hydraulically operated clutches for shifting the transmission to different speed ranges are operated by a novel solenoid-operated single rotary speed range selector valve. This rotary valve, which is movable to desired rotary positions by its solenoids, rapidly distributes pressurized hydraulic fluid to the appropriate clutch and eliminates the need for a separate individually actuatable solenoid valve for each transmission clutch. The rotary range selector valve and its drive means (including a geneva wheel and rack and pinion drive therefor which is actuated by upshift and downshift solenoid valves to rotate the range selector valve in step-by-step increments) insures that the transmission is shifted only one speed range at a time and insures that an engaged clutch for one speed range will be disengaged before another selected speed range clutch for another is engaged. Furthermore, blocker means are provided for preventing rotation of the geneva wheel and the range selector valve and thereby preventing engagement or disengagement of a clutch, unless the rack and pinion drive means for the geneva wheel is in a proper predetermined position. The arrangement offers many advantages, including reduced time between releasing one range clutch and engaging another, thereby enabling extremely fast shifts of the transmission.